This invention relates to inhalation apparatus for dispensing an inhalable substance and in particular, but not exclusively, to apparatus for use in the delivery of therapeutic substances to the human lung.
Medicinal inhalers are well known and have made a significant contribution to ailments such asthma. Of particular usefulness are hand-held metered dose inhalers and dry powder inhalers. Each produces an aerosol of fine particles containing medicament and which are carried into the respiratory system as a user inhales.
Several factors are known to effect the site at which deposition of such airborne particles are deposited in the respiratory system. Research has revealed that the electrostatic charge on the particles plays a very important part in determining the site of deposition and it has been shown that the level of electrostatic charge can be used to control particularly the site of deposition. A site may thereby be selected which is higher or lower in the bronchial tree to meet requirements of a particular therapeutic or diagnostic procedure. The level of charge can also serve to reduce the amount of particles lost through exhalation and this is particularly important where small quantities of medicament are delivered.
One means of achieving this is found in WO-A-94/19042 which describes dispensing apparatus for discharging a metered dose of a liquid in aerosol form from a pressurised dispensing container. The droplets are propelled through a passageway towards an inhalation port passing through a charging region.
The charging region contains one electrode which has at least one pointed feature and a second electrode having co-operating features of relatively low curvature. The aerosol emerging from the apparatus will carry an inherent level of electrostatic charge. The charge can be modified in a controlled manner by imparting further electrostatic charges to the particles as they pass through the charging region before being inhaled.
For certain applications it is desired to use electrostatic forces for generating an aerosol of electrically charged droplets particularly of a much smaller amount of liquid such as a single drop, which the above described apparatus would be unable to achieve.
EP-A-0224352 describes a method of generating a charged spray for ocular treatment. The formulation is supplied to a hollow spray nozzle which has an opening of such small cross-section as to retain up to 20 xcexcl of the formulation by surface tension. A metered dose of the formulation is supplied to the nozzle, after which a piston is activated to provide a current of air to force the formulation out of the nozzle. At the same time a high voltage is applied to a region of the nozzle in contact with the formulation causing the atomisation of the liquid to form a spray of electrically charged droplets for application to an eye.
This method thus requires the use of coordination of moving parts (the piston) and the triggering of the voltage.
U.S. Pat. No. 5,511,726 discloses a portable nebuliser capable of producing a finely divided aerosol having uniformly sized droplets. The nebuliser includes a source of fluid such as a capillary tube coupled to a fluid reservoir to which a high voltage is supplied in order to generate the aerosol by electrical atomisation. The nebuliser further includes a means for mechanical positive displacement of fluid control for controlling the amount of fluid atomised. One object of the present invention is to obviate the need for moving parts and to provide apparatus which is able to atomise a small quantity of liquid such as a single drop of liquid using electrostatic forces alone.
The invention therefore provides apparatus for dispensing an aerosol of electrostatically charged droplets comprising a housing in which are located a first electrode and a second electrode spaced from the first electrode, the apparatus further comprising means for delivering a metered quantity of liquid to an upper surface of the first electrode for atomisation and charging means for applying a higher potential to the second electrode with respect to the first electrode to effect atomisation, characterised in that the first and second electrodes are located in an open ended duct, the upper surface of the first electrode lying in a generally longitudinal plane of the duct, wherein the upper surface of the first electrode is shaped such that liquid delivered onto the upper surface of the first electrode spreads out over the surface of the first electrode causing a greater build-up of liquid around a perimeter of said first electrode than elsewhere on the first electrode.